1. Field of the Invention
The present invention pertains generally to a method for controlling delivery of radiation beams, and, in particular, to a method of controlling delivery of radiation therapy for medical purposes.
2. Description of the Related Art
Radiation therapy is a procedure frequently used in medicine, such as for shrinkage of tumors. Most small tumors are irradiated with high-energy photons. The radiation dose for each tumor and patient must be determined individually. In doing this one must determine from which direction and with which dose weight one is to give radiation treatment, given a known total dose that is to be dispensed.
An especially effective type of radiation therapy is the so-called pendulum irradiation, in which the source of rays traverses along a circular path in a space.
Today, planning the radiation procedure is usually performed manually by the treating physician, who determines both the direction of the single beam and the dose weight. The cross section of the beam used is generally rectangular or circular due to the use of motor-driven leaden jaws or round collimators.
It is, however, necessary to coordinate the shape of the beam with that of the tumor so that, with optimal and exact tumor radiation therapy, the tissue surrounding the tumor and/or the healthy organs are protected against significant exposure to ionizing radiation.
Toward that end, machines have been developed—so-called micro-multileaf collimators (MMLC's)—with which as many different field configurations as desired can be produced. This is accomplished by bringing into the beam path movable, leaf-like lamellae that are independent of each other.
Micro-multileaf collimators are also used with pendulum irradiation. At present it is customary to have up to four circular paths or oscillating motions in the traversed space. Raising the number of circular paths is, however, of crucial significance for increasing the effectiveness of the radiation therapy. Such an increase in the number of circular paths necessitates a considerable increase in the time and complexity of planning the radiation procedure. In part this is because the number of possible path combinations rises exponentially, requiring that the number of possible positions of the lamellae of the MMLC must also be considered. If an MMLC is also used for oscillating motions, the path of each lamella (for example, the number of lamellae may be 52) must also be calculated. It will be apparent, then, that the corresponding planning process is much too involved to rely on manual calculations.
U.S. Pat. No. 5,458,125 of the applicant herein discloses a process for the partial automatic calculation of the direction of the single beam and the weights for static radiation (i.e., no oscillating motion). This planning procedure is only suitable for conical collimators, i.e., for a circular cross section of the beam. The applicant herein has also described a procedure in which only static directions of the single beam are considered, that was published in the conference volume generated by the CAR Conference 97 (held 25-28 Jun. 1997 in Berlin, Germany). Additional prior art will be found in U.S. Pat. Nos. 3,987,281 and 4,868,843.
Planning that takes into consideration dose levels can no longer be carried out manually due to the many possible combinations when using MMLC with pendulum irradiation.